1. Field of the Invention
In general, the present invention relates to a display apparatus and a driving method for driving the display apparatus. More particularly, the present invention relates to a display apparatus employing light emitting units, which each have a light emitting device and a driving circuit for driving the light emitting device, and relates to a driving method for driving the display apparatus.
2. Description of the Related Art
As already known in general, there is a light emitting unit having a light emitting device and a driving circuit for driving the light emitting device. A typical example of the light emitting device is an organic EL (Electro Luminescence) light emitting device. In addition, a display apparatus employing the light emitting units is also already commonly known. The luminance of light emitted by the light emitting unit is determined by the magnitude of the driving current. A typical example of such a display apparatus is an organic EL display apparatus which employs organic EL light emitting devices. In addition, in the same way as a liquid-crystal display apparatus, the display apparatus employing the light emitting units adopts one of commonly known driving methods such as a simple matrix method and an active matrix method. In comparison with the simple matrix method, the active matrix method has a demerit that the active matrix method entails a complicated configuration of the driving circuit. However, the active matrix method offers a variety of merits such as a capability of increasing the luminance of light emitted by the light emitting device.
As already known, there are a variety of active-matrix driving circuits which each employ transistors and a capacitor. Such a driving circuit serves as a circuit for driving the light emitting device included in the same light emitting unit as the driving circuit. For example, Japanese Patent Laid-open No. 2005-31630 discloses an organic EL display apparatus employing light emitting units, which each have an organic EL light emitting device and a driving circuit for driving the organic EL light emitting device, and discloses a driving method for driving the organic EL display apparatus. The driving circuit employs six transistors and one capacitor. In the following description, the driving circuit employing six transistors and one capacitor is referred to as a 6Tr/1C driving circuit. FIG. 10 is a diagram showing an equivalent circuit of the 6Tr/1C driving circuit included in a light emitting unit located at the intersection of an mth matrix row and an nth matrix column in a two-dimensional matrix in which N×M light emitting units employed in a display apparatus are laid out. It is to be noted that the light emitting units are sequentially scanned by a scan circuit 101 in row units on a row-after-row basis.
The 6Tr/1C driving circuit employs a signal writing transistor TRW, a device driving transistor TRD and a capacitor C1 in addition to a first transistor TR1, a second transistor TR2, a third transistor TR3 and a fourth transistor TR4.
A specific one of the source and drain areas of the signal writing transistor TRW is connected to a data line DTLn whereas the gate electrode of the signal writing transistor TRW is connected to a scan line SCLm. A specific one of the source and drain areas of the device driving transistor TRD is connected to the other one of the source and drain areas of the signal writing transistor TRW through a first node ND1. A specific one of the terminals of the capacitor C1 is connected to a first power-supply line PS1 to which a reference voltage is applied. In the typical light emitting unit shown in the diagram of FIG. 10, the reference voltage is a reference voltage VCC to be described later. The other one of the terminals of the capacitor C1 is connected to the gate electrode of the device driving transistor TRD through a second node ND2. The scan line SCLm is connected to the scan circuit 101 whereas the data line DTLn is connected to a signal outputting circuit 102.
A specific one of the source and drain areas of the first transistor TR1 is connected to the second node ND2 whereas the other one of the source and drain areas of the first transistor TR1 is connected to the other one of the source and drain areas of the device driving transistor TRD. The first transistor TR1 serves as a first switch circuit connected between the second node ND2 and the other one of the source and drain areas of the device driving transistor TRD.
A specific one of the source and drain areas of the second transistor TR2 is connected to a third power-supply line PS3 to which a predetermined initialization voltage VIni for initializing an electric potential appearing on the second node ND2 is applied. The initialization voltage VIni is typically −4 volts. The other one of the source and drain areas of the second transistor TR2 is connected to the second node ND2. The second transistor TR2 serves as a second switch circuit connected between the second node ND2 and the third power-supply line PS3 to which the predetermined initialization voltage VIni is applied.
A specific one of the source and drain areas of the third transistor TR3 is connected to the first power-supply line PS1 to which the predetermined reference voltage VCC of typically 10 volts is applied. The other one of the source and drain areas of the third transistor TR3 is connected to the first node ND1. The third transistor TR3 serves as a third switch circuit connected between the first node ND1 and the first power-supply line PS1 to which the predetermined reference voltage VCC is applied.
A specific one of the source and drain areas of the fourth transistor TR4 is connected to the other one of the source and drain areas of the device driving transistor TRD whereas the other one of the source and drain areas of the fourth transistor TR4 is connected to a specific one of the terminals of a light emitting device ELP. The specific one of the terminals of the light emitting device ELP is the anode electrode of the light emitting device ELP. The fourth transistor TR4 serves as a fourth switch circuit connected between the other one of the source and drain areas of the device driving transistor TRD and the specific terminal of the light emitting device ELP.
The gate electrodes of the signal writing transistor TRW and the first transistor TR1 are connected to the scan line SCLm whereas the gate electrode of the second transistor TR2 is connected to a scan line SCLm-1 provided for a matrix row right above a matrix row associated with the scan line SCLm. The gate electrodes of the third transistor TR3 and the fourth transistor TR4 are connected to a third/fourth-transistor control line CLm.
Each of the transistors is a TFT (Thin Film Transistor) of a p-channel type. The light emitting device ELP is provided typically on an inter-layer insulation layer which is created to cover the driving circuit. The anode electrode of the light emitting device ELP is connected to the other one of the source and drain areas of the fourth transistor TR4 whereas the cathode electrode of the light emitting device ELP is connected to a second power-supply line PS2 for supplying a cathode voltage VCat of typically −10 volts to the cathode electrode. Reference notation CEL denotes the parasitic capacitance of the light emitting device ELP.
It is impossible to prevent the threshold voltage of a TFT from varying to a certain degree from transistor to transistor. Variations of the threshold voltage of the device driving transistor TRD cause variations of the magnitude of a driving current flowing through the light emitting device ELP. If the magnitude of the driving current flowing through the light emitting device ELP varies from a light emitting unit to another, the uniformity of the luminance of the display apparatus deteriorates. It is thus necessary to prevent the magnitude of the driving current flowing through the light emitting device ELP from being affected by variations of the threshold voltage of the device driving transistor TRD. As will be described later, the light emitting device ELP is driven in such a way that the luminance of light emitted by the light emitting device ELP is not affected by variations of the threshold voltage of the device driving transistor TRD.
By referring to diagrams of FIGS. 11A and 11B, the following description explains a driving method for driving an light emitting device ELP employed in a light emitting unit located at the intersection of an mth matrix row and an nth matrix column of a two-dimensional matrix in which N×M light emitting units employed in a display apparatus are laid out. FIG. 11A is a model timing diagram showing timing charts of signals appearing on the scan line SCLm-1, the scan line SCLm and the third/fourth-transistor control line CLm. On the other hand, FIG. 11B and FIGS. 11C and 11D are model circuit diagrams showing the turned-on and turned-off states of the transistors employed in the driving circuit. For the sake of convenience, in the following description, the scan period in which the scan line SCLm-1 is scanned is referred to as the (m−1)th horizontal scan period whereas the scan period in which the scan line SCLm is scanned is referred to as the mth horizontal scan period.
As shown in the timing diagram of FIG. 11A, during the (m−1)th horizontal scan period, a second-node electric-potential initialization process is carried out. The second-node electric-potential initialization process is explained in detail by referring to the circuit diagram of FIG. 11B as follows. At the beginning of the (m−1)th horizontal scan period, an electric potential appearing on the scan line SCLm-1 is changed from a high level to a low level but an electric potential appearing on the third/fourth-transistor control line CLm is conversely changed from a low level to a high level. It is to be noted that, at that time, an electric potential appearing on the scan line SCLm is sustained at a high level. Thus, during the (m−1)th horizontal scan period, each of the signal writing transistor TRW, the first transistor TR1, the third transistor TR3 and the fourth transistor TR4 is put in a turned-off state whereas the second transistor TR2 is put in a turned-on state.
In these states, the initialization voltage VIni for initializing the second node ND2 is applied to the second node ND2 by way of the second transistor TR2 which has been set in a turned-on state. Thus, during this period, the second-node electric-potential initialization process is carried out.
Then, as shown in the timing diagram of FIG. 11A, during the mth horizontal scan period, the electric potential appearing on the scan line SCLm is changed from a high level to a low level in order to put the signal writing transistor TRW in a turned-on state so that the video signal VSig appearing on the data line DTLn is written into the first node ND1 by way of the signal writing transistor TRW. During this mth horizontal scan period, a threshold-voltage cancelling process is also carried out. To put it concretely, the second node ND2 is electrically connected to the other one of the source and drain areas of the device driving transistor TRD. When the electric potential appearing on the scan line SCLm is changed from a high level to a low level in order to put the signal writing transistor TRW in a turned-on state, the video signal VSig appearing on the data line DTLn is written into the first node ND1 by way of the signal writing transistor TRW. As a result, the electric potential appearing on the second node ND2 rises to a level obtained by subtracting the threshold voltage Vth of the device driving transistor TRD from the video signal VSig.
The processes described above are explained in detail by referring to the diagrams of FIGS. 11A and 11C as follows. At the beginning of the mth horizontal scan period, the electric potential appearing on the scan line SCLm-1 is changed from a low level to a high level but the electric potential appearing on the scan line SCLm is conversely changed from a high level to a low level. It is to be noted that, at that time, the electric potential appearing on the third/fourth-transistor control line CLm is sustained at the high level. Thus, during the mth horizontal scan period, each of the signal writing transistor TRW and the first transistor TR1 is put in a turned-on state whereas each of the second transistor TR2, the third transistor TR3 and the fourth transistor TR4 is conversely put in a turned-off state.
The second node ND2 is electrically connected to the other one of the source and drain areas of the device driving transistor TRD through the first transistor TR1 which has been put in a turned-on state. When the electric potential appearing on the scan line SCLm is changed from a high level to a low level in order to put the signal writing transistor TRW in a turned-on state, the video signal VSig appearing on the data line DTLn is written into the first node ND1 by way of the signal writing transistor TRW. As a result, the electric potential appearing on the second node ND2 rises to a level obtained by subtracting the threshold voltage Vth of the device driving transistor TRD from the video signal VSig.
That is to say, if the electric potential appearing on the second node ND2 connected to the gate electrode of the device driving transistor TRD has been initialized at a level putting the device driving transistor TRD in a turned-on state at the beginning of the mth horizontal scan period by carrying out the second-node electric-potential initialization process during the (m−1)th horizontal scan period, the electric potential appearing on the second node ND2 rises toward the video signal VSig applied to the first node ND1. As the difference in electric potential between the gate electrode and the specific one of the source and drain areas of the device driving transistor TRD attains the threshold voltage Vth of the device driving transistor TRD, however, the device driving transistor TRD is put in a turned-off state in which the electric potential appearing on the second node ND2 is about equal to an electric-potential difference of (VSig−Vth).
Later on, a driving current flows from the first power-supply line PS1 to the light emitting device ELP by way of the device driving transistor TRD, driving the light emitting device ELP to emit light.
The process is explained in detail by referring to the diagrams of FIGS. 11A and 11D as follows. At the beginning of a (m+1)th horizontal scan period not shown, the electric potential appearing on the scan line SCLm is changed from a low level to a high level. Afterwards, the electric potential appearing on the third/fourth-transistor control line CLm is changed conversely from a high level to a low level. It is to be noted that, at that time, the electric potential appearing on the scan line SCLm-1 is sustained at a high level. As a result, each of the third transistor TR3 and the fourth transistor TR4 is put in a turned-on state whereas each of the signal writing transistor TRW, the first transistor TR1 and the second transistor TR2 is conversely put in a turned-off state.
During the (m+1)th horizontal scan period, a driving voltage VCC is applied to the specific one of the source and drain areas of the device driving transistor TRD through the third transistor TR3 which has been put in the turned-on state. The other one of the source and drain areas of the device driving transistor TRD is connected to the specific electrode of the light emitting device ELP by the fourth transistor TR4 which has been put in the turned-on state.
Since the driving current flowing through the light emitting device ELP is a source-to-drain current Ids flowing from the source area of the device driving transistor TRD to the drain area of the same transistor, if the device driving transistor TRD is ideally operating in a saturated region, the driving current can be expressed by Eq. (A) given below. As shown in the circuit diagram of FIG. 11D, the source-to-drain current Ids is flowing to the light emitting device ELP, and the light emitting device ELP is emitting light at a luminance determined by the magnitude of the source-to-drain current Ids.Ids=k*μ*(Vgs−Vth)2  (A)
In the above equation, reference notation μ denotes the effective mobility of the device driving transistor TRD whereas reference notation L denotes the length of the channel of the device driving transistor TRD. Reference notation W denotes the width of the channel of the device driving transistor TRD. Reference notation Vgs denotes a voltage applied between the source area of the device driving transistor TRD and the gate electrode of the same transistor. Reference notation C0X denotes a quantity expressed by the following expression:(Specific dielectric constant of the gate insulation layer of the device driving transistor TRD)×(Vacuum dielectric constant)/(Thickness of the gate insulation layer of the device driving transistor TRD)
Reference notation k denotes an expression as follows:k≡(½)*(W/L)*C0X The voltage Vgs applied between the source area of the device driving transistor TRD and the gate electrode of the same transistor is expressed as follows:Vgs≈VCC−(VSig−Vth)  (B)
By substituting the expression on the right-hand side of Eq. (B) into the expression on the right-hand side of Eq. (A) to serve as a replacement of the term Vgs included in the expression on the right-hand side of Eq. (A), Eq. (C) can be derived from Eq. (A) as follows:
                                                                        I                ds                            =                            ⁢                              k                *                µ                *                                                      (                                                                  V                        CC                                            -                                              (                                                                              V                            Sig                                                    -                                                      V                            th                                                                          )                                            -                                              V                        th                                                              )                                    2                                                                                                        =                            ⁢                              k                *                µ                *                                                      (                                                                  V                        CC                                            -                                              V                        Sig                                                              )                                    2                                                                                        (        C        )            
As is obvious from Eq. (C), the source-to-drain current Ids is not dependent on the threshold voltage Vth of the device driving transistor TRD. In other words, it is possible to generate the source-to-drain current Ids in accordance with the video signal VSig as a current flowing to the light emitting device ELP with a magnitude not affected by the threshold voltage Vth of the device driving transistor TRD. In accordance with the driving method described above, variations of the threshold voltage Vth of the device driving transistor TRD from transistor to transistor by no means have an effect on the luminance of light emitted by the light emitting device ELP.